Apparatus for the uniform distribution of fibers in an air stream

ABSTRACT

An apparatus for the manufacture of an air laid web in which individual cellulose fibers or textile fibers or their blends can be conveyed and distributed by air uniformly onto a forming zone composed of either a foraminous screen or a fibrous melt blown matrix on top of a consolidating vacuum box.

TECHNICAL FIELD OF INVENTION

This invention relates to an apparatus which will uniformly distribute individually deliberated cellulose or textile staple fibers so that they can be formed into a substrate or web or incorporated into another non-woven web of fibers.

BACKGROUND OF THE INVENTION

Historically, many attempts have been made at developing and commercializing apparatus for the formation and uniform distribution of air laid fibers, be it staple textile fibers or cellulose pulp fibers.

All of these apparatus have been cumbersome, highly complex mechanical devices which have had several disadvantages in their operations. Several of these devices have actually been commercialized for the formation of fibrous webs or substrates in the non-woven industry.

Air forming of wood pulp fibrous webs has been carried out for many years; however, the resulting webs have been used for applications where either little strength is required, such as for absorbent products—i.e., pads—or applications where a certain minimum strength is required but the tactile and absorbency properties are unimportant—i.e., various specialty papers. U.S. Pat. Nos. 2,447,161 to Coghill, 2,810,940 to Mills, and British Pat. No. 1,088,991 illustrates various air-forming techniques for such applications.

In the late 1940's and early 1950′s, work by James D'A. Clark resulted in the issuance of a series of patents directed to systems employing rotor blades mounted within a cylindrical fiber “disintegrating and dispersing chamber” wherein air-suspended fibers were fed to the chamber and discharged from the chamber through a screen onto a forming wire—viz., J. D'A. Clark U.S. Pat Nos. 2,748,429, 2,751,633 and 2,931,076. However, Clark and his associates encountered serious problems with these types of forming systems as a result of disintegration of the fibers by mechanical co-action of the rotor blades with the chamber wall and/or the screen mounted therein which caused fibers to be “rolled and formed into balls or rice which resist separation”—a phenomenon more commonly referred to today as “pilling”. Additionally, J. D'A. Clark encountered problems producing a web having a uniform cross-direction profile, because the fiber input and fiber path through the rotary former was not devoid of cross flow forces.

The formation of non-woven webs emanate from the textile industry as a result of taking a very old process such as carding, and combing the textile fibers into a wide web of loose fibers after which they are bonded either chemically or thermally into a consolidated substrate or web. The distribution of these fibers is done mechanically through a series of combing steps in which saw toothed clothed rolls work the fibers into individual strands from clumps and spreads them in the process to form a web. These types of processes lend themselves primarily to textile staple fibers that have fiber lengths of 1 to 2 inches. Even though further evolution of this process has led to the use of air to assist the doffing of the fibers off the main cylinder and in forming a web, these processes do not lend themselves well to short cellulosic fibers which are typically in the 2 to 3 mm in length.

In the mid nineteen sixties and seventies, a combination air and mechanical carding process took the technology further by taking combinations of short cellulosic fibers and longer textile staple fibers combining them mechanically and then air conveying them into a forming chamber so that they could be made into a substrate or web. U.S. Pat. Nos. 3,982,302, 4,004,323 belonging to Scott Paper describe this process.

The disadvantage of this process was the fact that it was limited to the amount of short cellulose fibers that it could handle. The longer textile staple fibers were still needed to provide an adequate entanglement and matt structure that would allow to be combed or picked into an air stream for forming.

Both of the carding based processes described so far are depending on the basis weight cross direction profiles of the fibrous matt leading to the forming device. These cross direction profiles are developed and formed prior to the forming step and are somewhat fixed. So if they are not adequate there are no means of correcting of adjusting for them during the formation process. What the former sees in cross direction basis weight profile, the substrate or web will get as a result.

A second type of system for forming air-laid webs of dry cellulosic fibers which has found limited commercial use has been developed by Karl Kristian Kobs Kroyer and his associates as a result of work performed in Denmark. Certain of these systems are described in: Kroyer U.S. Pat. Nos. 3,575,749 and 4,014,635; Rasmussen 3,581,706 and 3,669,778; Rasmussen et al. 3,769,115; Attwood et al. 3,976,412; Tapp 4,060,360; and, Hicklin et al. 4,074,393.

This type of sifting equipment suffers from poor productivity especially when making light weight webs. For example, the rotor action concentrates most of the incoming material at the periphery of the blades where the velocity is at a maximum. Most of the sifting action is believed to take place in these peripheral areas, while other regions of the sifting screen are either covered with more slowly moving material or are bare. Thus, a large percentage of the sifting screen area is poorly utilized and the system productivity is low. Moreover, fibers and agglomerates tend to remain in the forming head for extended periods of time, especially in the lower velocity, inner regions beneath the rotor blades. This accentuates the tendency of fibers to roll up into pills.

In an effort to overcome the productivity problem of such systems, complex production systems have been devised utilizing multiple forming heads—for example, up to eight separate spaced forming heads associated with multiple hammermills and each employing two or three side-by-side rotors. The most recent sifting type systems employing on the order of eighteen, twenty or more rotors per forming head, still require up to three separate forming heads in order to operate at satisfactory production speeds—that is, the systems employ up to fifty-four to sixty, or more, separate rotors with all of the attendant complex drive systems, feed arrangements, recycling equipment and hammermill equipment.

Honshu, U.S. Pat. Nos. 3,984,898 and 4,160,059, at approximately the same time developed a different concept to the above by combining the fiberization or defibration step into one single step. In this manner the cross direction of the web was dependent on the pulp lap cross-direction profiles feeding the defibrator. The function of the air stream was only to convey the individual fibers onto the foraminous screen to form the web. This process had several disadvantages, as the air stream employed for web forming could not be properly psychometrically conditioned, impacting the quality of the web due to static clumping as a result of very dry fluff fibers.

During the 1970's a series of patents were issued to C. E. Dunning and his associates which have been assigned Kimberly-Clark; such patents describing yet another approach to the formation of air-laid dry fiber webs. Such patents include: Dunning U.S. Pat. Nos. 3,692,622, 3,733,234 and 3,764,451; and, Dunning et al. 3,776,807 and 3,825,381. However, this system requires preparation of pre-formed rolls of fibers having high cross-directional uniformity and is not suitable for use with bulk or baled fibrous materials, such that, to date, the system has not found a commercial application.

Kimberly Clark also developed another forming process that is described in their U.S. Pat. No. 4,100,324 in which defibrated cellulose pulp is air formed into a molten microfiber meltblown polypropylene stream to form an air laid web without the use of chemical binders. The process described uses the defibrator as the method of conveying the fibers in an air stream into the polypropylene matrix. It is handicapped by the fact that it is a combination defibrator and air former which does neither function well. It is a highly mechanical device which limits the width of the machine based on the width of the defibrator which must span the entire width of the former. The critical speed of the defibrating rotor is the limiter on this property limiting the width typically to below two meters.

Celli in US Application 20060174452, a few decades later took the same concept as the Kroyer distributor, but re-designed the geometry of the rotors. Rather than having the rotors rotate in the cross direction with their blades parallel to the distributor screens and creating a cross machine direction race track fiber flow inside the distributor, these rotors being cylindrical and rotating in the machine direction with parallel axes, perpendicular to the flow and equipped with radial elements in the form of needles or rods.

Dan Web in U.S. Pat. No. 4,278,113, 4,352,649, 4,640,810, 5,885,516 and 7,107,652 in an attempt to differentiate themselves from the Kroyer distributors in which they claimed parallel interfaces between the distributor screen geometries and the foraminous forming screen, developed a similar concept distributor but in a round drum-shaped geometry. This former head, where a fiber material mixed with air is conducted to at least one rotating perforated drum in a former head by injection, has internally fluidizing means constituted by air nozzles arranged longitudinally of the drum with the air being controlled longitudinally. Again, in this case the cross direction distribution of fibers is accomplished by the trajectory of the fibers inside the rotating drum formers, and the air system's primary purpose was to convey the fibers to the forming screen.

Indeed, heretofore it has not been believed that air-forming techniques can be advantageously used in high speed production operations to prepare cellulosic sheet material that are sufficiently thin, and have adequate cross-directional profiles to satisfy the performance requirements of the final product application.

BRIEF SUMMARY OF INVENTION

This invention is for a device to air lay cellulose, textile staple fibers and blends thereof by taking these fibers from an air transported duct and spreading them uniformly so that they can be air laid into a forming zone and consolidated into a fibrous web. This device is aerodynamically designed and has no moving parts making it an elegantly simple and effective forming device compared to prior art.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 illustrates the tapered cross sectional profile of the forming funnel with its decreasing width to maintain and/or accelerate the fibers and air flows through the unit.

FIG. 2 illustrates a view of the forming funnel with its three primary components, the insertion of fiber, the spreading of the air and fiber flow and the angled piece for control of the cross machine basis weight profile prior to conveyance into the forming zone.

FIG. 3 illustrates a tandem construction of two fiber forming modules with the unitary angled piece.

FIG. 4 illustrates the cross section of the angle unit discharge piece with the adjustment plate and screws which are used to control the cross machine basis weight profile of the fiber prior to conveyance to the forming zone.

FIG. 5 illustrates the air flow profiles at the outlet of the angled discharge section.

FIG. 6 illustrates the layout of the fiber forming in relation to the forming zone.

DETAILED DESCRIPTION OF THE INVENTION

This invention simplifies what has been attempted before in a very elegant aerodynamic execution of a device which not only distributes the fibers uniformly in the cross machine direction, but also allows them to be formed into a web.

It is well known and published in the art what the key aerodynamic parameters for conveying solid particles in an air stream. The difficulty has been in developing a device that can maintain these conditions and distribute fibers onto a forming zone.

A forming zone on most air laid machines is a foraminous screen supported over a vacuum table to consolidate the individual fibers into a web. Other forming zones are rotary vacuum drums or condensers into which the air is blown into and the fibers are matted into a web on its surface later to be transferred to another process operation. Other forming zones are composed by air conveying the individual cellulose fibers into a curtain of molten polymeric fibers as they are extruded from the die and later consolidated in a blended form onto a forming screen. FIG. 6 shows a typical installation of this device, items 60 and 70, with in association with a forming zone comprised of a forming vacuum box with a foraminous forming screen item 90, as well as a meltblown extrusion die identified by item 80.

Fibers or particles, because they are denser and consequently heavier than air, tend to follow their own trajectories due to the iso-kinetic forces exhibited in the air stream. Therefore, it is imperative that air forming devices be designed to accommodate not only for the air characteristics required, but also accommodate the ability to uniformly convey and distribute particles or fibers in the cross direction, especially when a substrate or web is to be formed from the device.

Fibers, especially cellulose fluff fibers, need to be well defibrated into individual fibers. This process is .well understood in the industry, with several successful designs currently in the market place. Companies like Kamas, M&J, and Famecannica have developed devices to defibrate pulp into individual fibers for many years now. The biggest use of these fibers is in absorbent cores for disposable products such as baby diapers and feminine care sanitary products. Fibers from such devices can then be conveyed by air to their final fluff forming devices.

In the case of forming absorbent batts in which the thickness or basis weight of the batt is large (greater than 100 gsm) the aerodynamic characteristics of the fluff forming devices are not as critical. The aerodynamic and design characteristics of the forming device become much more critical when the requirement is to form a substrate of less than 100 gsm and closer to the 20 gsm level. The challenge becomes on taking fibers that are being conveyed in a round duct at velocities that are typically in the 1000 to 10,000 fpm range and spreading these fibers to widths up to five meters wide while achieving a uniformity of the fibers or particles ranging under +/−10% by accepted standard test methods used in measuring this parameter.

The present invention uses sound engineering principles in achieving this goal. The critical parameter of this invention is to take fibers from a circular duct and spread them to widths of approximately 1.5 to 3.2 meters or greater uniformly.

FIG. 1 shows the device which accomplishes this goal. It is a funnel like device which is fed by a round duct conveying fibers in an air stream.

Prior to the introduction of fibers into the air stream the spreading and forming device needs to provide air flows at the discharge which are extremely uniform in the cross direction. This is accomplished by maintaining constant or accelerating velocities through the funnel length, as the area at the discharge of the round duct is the same or slightly greater than the rectangular opening at the discharge of the funnel. This concept of maintaining constant or slightly accelerating air velocities through any cross sectional plane such that AA=>BB=>CC=>DD as shown in FIG. 1 items 10, 20, 30, and 40 of the spreading device is critical in achieving uniform cross direction air profiles at the discharge of the unit.

FIG. 4 shows the air profiles that are achieved applying these techniques to the forming device. This data was obtained from an unmodified discharge nozzle profile. It can be basically made flat when the profile control system shown in FIG. 5 is implemented.

The second key parameter is to have the fiber velocities which are equivalent to the air velocities of the conveying air stream be dissipated so that the iso-kinetic energy of the fiber is greatly reduced as it enters the spreading device. This is accomplished by the geometry of item 50 of FIG. 2, which shows the round duct entering the funnel at an angle, thus having the fibers hit the far wall of the spreading device. In this manner the velocity of the fibers and the momentum of the fibers are dissipated. This allows the fibers then to be re-aligned with the airflow profiles that will be developed by the geometries used in the design of the spreading device.

If this step is not done, the fibers would have the tendency to stay in the center of the device creating a heavier center on the substrate formed. The angle of the circular duct can vary, as long as the fiber velocity is dissipated as they strike the back wall of the spreading and forming device. Other means of conveying the fibers to the entrance of the forming device can be contemplated so that the velocities of the individual fibers align themselves with the velocities of the air stream.

Once the fibers are in the spreading and forming device, it is important that they have enough residence time in the device to streamline themselves to the airflows that have been developed within the device. This is accomplished by having the length of the device be at a minimum equivalent to ten times the diameter of the round feed duct for the fibers. Lengths much shorter than 10 equivalent diameters will result in less efficient fiber spreading in the cross direction and worse profiles.

The third key element of this invention is the ability to control the discharge of the fibers onto a foraminous forming screen or onto another fiber stream in order for the fibers to blend with these fibers forming a web.

In this case the angle in which the fibers are directed onto either a forming zone, or is critical. This angle may require adjustment. Item 60 in FIG. 2 shows a device which is used at the discharge end of the spreading device to turn the fibers in the proper direction. The figure shows a nozzle with 90° turn. This angle is adjustable and can be adjusted to be whatever the application requires it to be. Another method that can be used to adjust for the angle is to tilt the spreading and forming device to that angle which is also required for proper web forming.

Another advantage that this system has is the ability to have modular forming heads. Thus, they can be combined individually in the cross machine direction making the formation width of the machine a non-issue. FIG. 3 shows the advantage of this design by showing two side to side formers. There is no limitation to the number of formers that can be combined in the cross machine direction making it possible to achieve widths of five meters or more. For practical purposes the ideal width of the formers are in the range of 1 to 1.5 meters. Even though the fiber formers are separate, the discharge portion, item 60 in the figures shown, is a continuous piece. In this manner, the fibers are air conveyed in a uniform cross direction manner to the forming zone without any separation as a result of the separate conveying funnels.

Furthermore, the discharge section as is shown in FIG. 5, item 60, has an adjustable bottom plate, item 61, which can be constricted in opening by adjustable screws to influence the trajectory of both the fiber and air stream. This added control system will guarantee a uniform profile of fibers into the forming zone. 

1. A device for air forming a fibrous web by expanding the width of the air conveying duct to the appropriate forming zone width and maintaining adequate cross direction fiber basis weight profiles by: a. dissipating the fiber velocities at the entrance of the forming device so that they are less than the conveying air stream at the top of the forming device, b. having a geometry in the forming device which provides constant or slightly accelerating air and fiber velocities in the device to those found in the conveying duct, and, c. having the length of the spreading device be about ten times greater than the diameter of the incoming duct.
 2. A means to adjust the cross-directional gap at the discharge of the device in claim 1 to control the cross-direction air and fiber discharge onto a forming zone, by restricting the flow through undulations on a static plate.
 3. A forming apparatus made through a combination of the device in claim 1 composed by joining in the cross-direction individual devices of claim 1 to create a forming device of much greater width. 